Moving head lighting fixtures are commonly known in the art of lighting and especially in connection with entertainment lighting. Typically a moving head light fixture comprises a head having a number of light sources which creates a light beam and number of light effect means adapted to create various light effects. The head is rotatable connected to a yoke and the yoke is rotatable connected to a base and the result is that the head can be paned and tilted whereby the light beam can be directed in many directions. Typically moving head light fixtures allows an angular position of the yoke in relation to the base between 0-540 degrees, meaning that the yoke can be rotated 1½ revolution in relation to the base. Similar, typical moving head light fixture allows the head an angular position of the head in relation to the yoke between 0-270 degrees. However some moving head light fixtures allows larger or smaller ranges of angular displacement of the yoke or head.
As a consequence most moving head light fixtures are provided with means for registering the position of the yoke in relation to base and the position of the head in relation to the yoke. Traditionally this have been carried out by resetting the yoke and head position at startup by rotating the head and yoke to the end stop position and there're after adapt the processor of the moving head to count the number of steps which the step motors have turned in relation to the end stop position. However there are risks that step motors loses steps during operation which results in the fact that the counting becomes inaccurate. This has been solved by providing optical sensors adapted to count rotation steps of the axis of rotation instead of counting the steps applied by the step motors.
For instance U.S. Pat. No. 4,716,344 discloses a motorized lamp directing device where the lamp housing is rotatable mounted in a U-shaped braked and can be rotated by a motor assembly. The motor assembly include a box-like enclosure having four mutually perpendicular side walls extending downwardly from a top plate, where a DC motor and its gear box are mounted on the top plate. An input shaft extends into the interior and cooperates with an endless belt entrained about a larger diameter pulley secured to a driven shaft. The drive shaft is connected to and rotates the lamp in relation to the U-shaped bracket. The relative diameters of the input shaft and the pulley of are 18:120. Secured to the end of the input shaft is an encoder wheel having a pattern of sensible indicia inscribed thereon and capable of being sensed by an optical sensor.
In is known that the optical sensors may be disturbed by smoke or dust and there are thus are a risk of losing counts, whereby the determined position is in correct.
US2003/0137842 discloses a reflective mechanism for a stage lamp providing an incident light. The reflective mechanism includes a mounting device, a rotary device, a rotary frame, a reflective device rotatably mounted to the rotary frame, and a transmission device. The rotary device is mounted to the mounting device and comprises a fixed outer ring, a middle ring concentrically, rotatably mounted in the fixed outer ring, and an inner ring concentrically, rotatably mounted in the middle ring. The inner ring and the middle ring are driven by a first power device and a second power device, respectively. The rotary frame is attached to the middle ring to turn therewith. The transmission device includes a first transmission member mounted to the inner ring to turn therewith and a second transmission member that is mounted to the reflective device to turn therewith and that is connected to the first transmission member. A light source generating a light beam is provided in the mounting device, the light beam is passing through the outer, middle and inner ring and hits the reflection device and is thereby reflected in a desired direction as the reflection device can be rotated and tilted.
A magnetic element is mounted to a side of the middle ring, and another magnetic element is mounted to a side of the inner ring. Each magnetic element is detected by an associated sensor, which thereby detect the position of the middle ring and inner ring in relation to the mounting device, thereby providing a zeroing function at the beginning of starting of the power devices. The magnetics elements acts as a reset mechanism and makes it only possible to detect the position of the reflective device it is arranged at a certain position in relation the mounting devices and it is thus not possible to detect the present position of the refection device. Further it is not possible to detect the angular position of the reflection device in relation to mounting device and it is thus not possible to differentiate between different resolutions of the refection device.
EP 2 103 864 and EP 2 103 865 (corresponds to US 2009/0231852 and U.S. Pat. No. 7,654,692) disclose as moving head light fixture comprising at least one absolute encoding device. The absolute encoding device indicates the angular movement of a first input axel in relation to the encoding device and comprises a gearbox. The gear box comprises the first input axel, a first toothed timing wheel driven by the first input axel, a second toothed timing wheel intermeshing the first toothed timing wheel and a second axel connected to and rotated by the second toothed timing wheel. The number of tooth of the first and second timing wheels is different and the first input axel and the second axel timing wheels rotates thus at different angular velocity. Magnets generate a magnetic field mostly perpendicular to the longitudinal direction of the axles and are arranged at the ends of the first input axel and the second input axel. The magnets face a magnetic encoding device capable of measuring the direction the magnetic field whereby the angular position of the axel can be determined
The first input axel of the absolute encoding device is driven by a belt. The belt is further interconnecting the rotation axis of the head of a moving head light fixture and a motor, where the motor is adapted to drive the belt, which then rotates the head. The first input axel of the absolute encoding device is rotated as the belt rotates and the angular position of the first input axel and the second axel in relation to each other changes when the belt is driven. The internal processor uses the magnets and magnetic encoders to sense the position of the first input axel and the second axel in relation to each other and can based on these measurements determine the absolute position of the head. This set up makes it possible to immediately to determine the position of the head when the moving head is turned on and the measurement it further not disturbed by dust and smoke. However the absolute encoding device must be very accurately calibrated in connection with the manufacturing process as the internal processes need to know the correlation between the different angular positions of the first input axel and the second axel of the absolute encoding device and the actual position of the head. Such process makes the manufacturing process more complicated and expensive. Further there is a risk that a person may move the belt and first axes in relation to each other during service inside the moving head light fixture and as a consequence the factory calibration of the moving head is destroyed.
Further the absolute encoding device is rather expensive to manufacture as it comprises many parts and needs to be provided separately. Further the magnetic encoders are relatively expensive and a moving head where such absolute encoding device have been used to determine yoke position in relation to base and head position in relation the yoke need at least four of such magnetic encoders.
The competition in the market has traditionally been based on the optical performance of the moving head such as light output, number of light effects, color mixing etc. The competition in the market has lately changed such that parameters such as quality, serviceability and price have become the most important factors. There is thus a need for a competitive moving head lighting fixture with regard to quality, serviceability and price. The prior art moving heads comprise many components and are thus rather complicated to manufacture which increases the price of the moving head and further complicates the serviceability of the moving head.